A Grand Enigma

The Grand Canyon in Arizona is not the world's deepest canyon, nor
is it the widest, but it is an icon of erosion—of the
ability of rivers to shape landscapes and transport sediments from
mountains to the sea. This gash carved by the Colorado River across
the southern Colorado Plateau invites the controversy swirling about
it, which is generated on the one hand by geologists eager to
unravel the record of erosional processes so prominently on display,
and on the other hand by creationists wanting to exploit the canyon
to further their explanations of the Earth's origins. James Lawrence
Powell's Grand Canyon, although not flawless, belongs on
the shelf of any reader serious about understanding not just
scientific explanations of the canyon's origin but the entire
Colorado Plateau and its geological history.

The book is reasonably well written and provides an unusually lucid
history of geological thinking, particularly with respect to river
geomorphology and the origin of landforms. The account is marred by
repetition, however, and the treatment of John Wesley Powell's
explorations and ideas is almost as circuitous as the path of the
Green River in Labyrinth Canyon. If the author's intention was to
revisit Major Powell and his legacy within the context of the
canyon's origins, the book is an abject failure. That John Wesley
Powell overcame his humble academic background to make large
contributions to geological understanding of rivers and landscapes
is not news, nor is his political acumen or his ability to judge
scientific talent. Given that exemplary biographies of Powell
already exist—Wallace Stegner's Beyond the Hundredth
Meridian (1954) and Donald Worster's River Running
West (2000)—the first third of this story could have been
told in less space.

Grand Canyon shines in its discussion of Powell's
protégés: Grove Karl Gilbert and Clarence Edward
Dutton, and those who followed in the 20th century. The perspectives
of Gilbert (that these are landforms inexorably ground by natural
machinery) and Dutton (geomorphology as poetry) are eloquently
presented in this book. The progress of scientific thought is
illustrated by its successes and failures; even stalwarts such as
Gilbert got it wrong on occasion, as when he concluded that Meteor
Crater in northern Arizona had been created by steam volcanism
rather than meteorite impact.

The evolution of thought from the first conference on Grand Canyon
geology in 1964 to the second one in 2000 is well explained. The
1964 report, spearheaded by legendary Grand Canyon geologist Edwin
McKee, concluded that a river cut upstream through the landscape,
working its way northward from the Gulf of California and then
eroding through the Grand Wash Cliffs, where it linked up with the
streams of the Hualapai drainage system, setting the location that
the Grand Canyon would later occupy. Eventually, the headward
erosion breached the Kaibab Plateau and captured the ancestral upper
Colorado River, an action known as stream piracy. Nowadays this is
one of the less likely of several possible explanations as to the
origins of the canyon, all of which focus on the impediment that the
Kaibab Plateau posed and the means by which the river surmounted it.

Most participants in the 2000 conference supported a relatively
young age (4 to 5 million years) for the Grand Canyon on the basis
of both upstream and downstream evidence. Although many diverse
ideas remain about how the canyon formed, two stand out. One holds
that, long before the Grand Canyon existed, the ancestral Colorado
River breached the Kaibab Plateau, turned 90 degrees and flowed
north to northwest. Headward erosion from the Grand Wash Cliffs
ultimately intercepted the river, which then downcut to its present
depth. If this is what happened, there should be Colorado River
gravels northwest of the canyon, but none have yet been found. So
this explanation lacks some critical evidentiary support.

An intriguing alternative theory holds that the Kaibab Plateau
served as a natural dam, forming a large lake to the east covering
much of the present-day Navajo Indian Reservation. Breaching of that
dam, either through overtopping or by dissolving the Paleozoic
limestones to form pipelike channels, developed a large enough
sustained flow to carve the canyon from upstream. Again the critical
evidence supporting this idea is missing, perhaps having been washed
downstream over the past 4 to 5 million years.

Geological reasoning and technological innovation have combined to
hone questions about the canyon's origin, even as answers have
become more elusive. This book showcases the development of the
geosciences by focusing on one massive landform and the history of
its interpretation; seldom has this history been presented in such a
readable and entertaining form.

His choice of subtitle notwithstanding, Powell has written a book
that shows that the Grand Canyon, with its tantalizing testimony
concerning Earth's history, remains an unsolved puzzle, despite a
wealth of specific knowledge about its origins. Interpreting the
reasons for missing rock is a perilous endeavor, and it may be that
so much evidence of the geological history of the Colorado River is
gone that a consensus answer as to the origin of the Grand Canyon
will never emerge. Powell's eloquent retrospective on the
development of geological thought reminds us of one fundamental
truth: Scientists disprove hypotheses rather than prove them. The
present-day theories reviewed in this book may well be discredited
in the future.

The belief of some geologists that there is a final answer to the
question of how the Grand Canyon came to exist contrasts sharply
with the attitude of most practitioners that scientific
understanding is always subject to revision as new evidence
accumulates. As the author notes, those who wrote the 1964 McKee
report succumbed to the perceived need for a conclusion about the
canyon's origin instead of embracing the uncertainty. This book
abundantly shows that the elegant scientific knowledge developed by
those attempting to extract the secrets of the canyon's origin is
itself the prize. As the saying goes, sometimes the journey is
better than the destination.